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<bit>: Use popcount for has_single_bit() if it's always available #5534

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Merged
merged 4 commits into from
May 28, 2025
Merged

<bit>: Use popcount for has_single_bit() if it's always available #5534

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53ce129
`<bit>`: Use popcount for `has_single_bit()` if it's always available
pps83 May 23, 2025
dda8819
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pps83 May 23, 2025
420c391
Always define `_POPCNT_INTRINSICS_ALWAYS_AVAILABLE`, leave `_HAS_POPC…
StephanTLavavej May 28, 2025
9f7ee26
Avoid code duplication.
StephanTLavavej May 28, 2025
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5 changes: 2 additions & 3 deletions stl/inc/__msvc_bit_utils.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -281,7 +281,8 @@ _NODISCARD int _Checked_x86_x64_countr_zero(const _Ty _Val) noexcept {
#define _POPCNT_INTRINSICS_ALWAYS_AVAILABLE 0
#endif // ^^^ intrinsics not always available ^^^
#else // ^^^ intrinsics available / intrinsics unavailable vvv
#define _HAS_POPCNT_INTRINSICS 0
#define _HAS_POPCNT_INTRINSICS 0
#define _POPCNT_INTRINSICS_ALWAYS_AVAILABLE 0
#endif // ^^^ intrinsics unavailable ^^^

#if _HAS_POPCNT_INTRINSICS
Expand Down Expand Up @@ -384,9 +385,7 @@ _CONSTEXPR20 decltype(auto) _Select_popcount_impl(_Fn _Callback) {
return _Callback([](_Ty _Val) _STATIC_LAMBDA { return _Popcount_fallback(_Val); });
}

#undef _HAS_POPCNT_INTRINSICS
#undef _HAS_TZCNT_BSF_INTRINSICS
#undef _POPCNT_INTRINSICS_ALWAYS_AVAILABLE

_STD_END

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6 changes: 6 additions & 0 deletions stl/inc/bit
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Expand Up @@ -84,6 +84,12 @@ _NODISCARD constexpr int countl_zero(_Ty _Val) noexcept;

_EXPORT_STD template <_Standard_unsigned_integral _Ty>
_NODISCARD constexpr bool has_single_bit(const _Ty _Val) noexcept {
#if _POPCNT_INTRINSICS_ALWAYS_AVAILABLE
if (!_STD is_constant_evaluated()) {
return _Unchecked_popcount(_Val) == 1;
}
#endif // ^^^ _POPCNT_INTRINSICS_ALWAYS_AVAILABLE ^^^

return (_Val ^ (_Val - 1)) > _Val - 1;
}

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